Another simple and commonly used probe in plasma diagnostics is the retard-ing field energy analyzer (RFEA). It collects an ion saturation current over a potential sweep, while constantly repelling electrons. The RFEA probe is typically used to find the ion velocity distribution, the ion energy distribution and the ion temperature from a plasma beam [37–40].
4.2.1 Principles
The RFEA probe consists of several small grids placed inside an outer, cylin-drically shaped, housing. The housing is usually either a metal structure floating at the ground potential or made of some insulating material, e.g.
ceramics, leaving it at the plasma potential; the latter is often preferred as it does not perturb the plasma as much [37]. The front plate of the probe, i.e.
the part facing the plasma beam, has a small aperture in the middle, often covered by a grounded grid. Inside there are typically two or three grids, sep-arated by insulated rings, and a collector plate. The first grid is the repeller grid and is biased strongly negative to repel and stop electrons from entering the aperture. The second is the discriminator grid which is biased with a time-depended sweepVg from a negative to a positive voltage, similar to that used in the Langmuir probe, however only ions with a parallel kinetic energy higher than Vg are able to pass through. If one chooses to apply a third grid
4.2. RFEA PROBE|41
it is commonly biased positively and used to stop any electrons that may have escaped through the front grid, and to contain the secondary electrons sputtered from the collector as high energy ions collide into it. The collector plate is often applied with a weak negative bias to attract the incoming ions [37, 39].
The probe characteristics of an RFEA produce an IV-curve with an ion saturation current and no electron current. Typical RFEA IV-curves (figure 6 from [38]) can be seen in figure 4.3 for different pressure values).
Figure 4.3: Typical IV-curves produced by an RFEA probe for different values of pressure (figure 6 from [38]).
The collected ion saturation current is given by
I(v0) = Ae Z ∞
v0
vf(v)dv (4.7)
where A is a constant depending on the front plane pin-hole, v is the ion velocity and f(v) is the ion velocity distribution function or the ion energy distribution function. v0 is the minimum velocity and is given as [37, 39]
v0 =
r2eVg
mi (4.8)
wheremi is the ion mass. From equations (4.7) and (4.8) it is clear that the velocity distribution function is proportional to the derivative of the collected ion current with respect to Vg. Using this we find an expression for the ion velocity distribution function [39]
The RFEA probe used in this experiment consists of a cylidrical ceramic housing with a front plate, three grids and a collector plate inside. The front plate is made of stainless steel and has an aperture with a diameter dap = 4 mm. Behind it is the front grid, which should be grounded but is not connected to anything due to faulty wires in the probe. The probe would work well enough for our purposes without it, so we did not spend time trying to fix this. Then we have the repeller grid, this is applied with a voltage Vr = 70 V and is there to repel electrons. The last grid is the discriminator with a time-dependent potential bias applied, similar to the Langmuir probe. The collector gather the ion current of the ions able to pass through the sweeping potential. All grids and the collector are made of stainless steel, and all the grids have a transparency ofTg ≈0.5.
A schematic of the RFEA probe can be seen in figure 4.4 (adapted from figure 3 in [37]). Both the sweep signal and the incoming probe signal go through two separate amplifiers. The collector is biased to Vcollector =−9V using a battery pack.
The probe is installed in the same way as the Langmuir probe, on the thouroughfeed with an L-shaped joint, however the probe is 10 cm shorter than the Langmuir probe, hence it is placed at a distance at 20 cm away
4.2. RFEA PROBE|43
Figure 4.4: Sketch of the RFEA probe with the different components and materials (adapted from figure 3 in [37]).
from the plasma source.
Chapter 5
Results and Discussion
In this chapter the results obtained in the experiment are presented. The plasma parameters are derived and discussed with reference to the theory reviewed in chapters 2 and 4, along with their significance regarding the Space Simulation Chamber (SSC).
5.1 Paschen curve
In equation 2.12 we see the breakdown criteria for a plasma discharge to happen in a specific gas species as a function of pressure. Measurements were taken at different pressures where the voltage needed to make breakdown happen was noted. Two data sets were recorded and can be seen plotted in figure 5.1.
For the x-axis only the pressurepwas used instead of the traditional product pd, where d is effective distance between cathode and anode. This is due to the complex geometry of our plasma source, where we have a central cathode and a much larger cylidrically shaped anode. From the results in figure 5.1 we see a curve shaped as expected compared to the theoretical one from figure 2.2.
The plasma is relatively reproductive as seen from the consistency of the
Figure 5.1: Two sets of experimentally measured values for the Paschen curve for an argon gas discharge in the ion source. A moving average filter and a Savitzky-Golay filter has been applied to the curves for smoothing.
two data sets, which is a major goal in building a Space Simulation Cham-ber. A few things to note is that when breakdown happens the discharge grows exponentially, and since the values for the breakdown voltage are be-ing manually adjusted and recorded the measurements ofVb should have an error of about ±5 V. The pressure sensors has an estimated error of about
±0.5·10−5 mbar.